When it comes to making repairs, the more detail engineers can get before an astronaut is sent out on a spacewalk to fix it, the better—that’s where three-dimensional imaging comes in. A Florida company called Photon X has developed a sensor that extracts 3D information from a single still image; it’s generally used for tasks like facial recognition and, soon, in some medical exams. Once a 3D camera is certified for spaceflight, it will be an essential inspection tool.
Cameras can inspect damage, but it’s unlikely they will catch the impact as it happens. So to monitor for leaks in the station walls, ultrasonic and piezoelectric (pressure-detecting) sensors will listen for specific frequencies or measure changes in the station’s internal atmosphere. NASA is testing a prototype system that uses eight to 12 sensors, Studor says, which are being calibrated to the normal background noise the station generates all the time.
Detecting a breach immediately is critical. The station’s heavy shielding includes many areas covered in multiple layers of aluminum called Whipple shields, as well as Nextel and Kevlar fabric blankets. These are designed to absorb at least some of the energy from an impact, but if the object is large enough, it can penetrate the layers and ignite a fire when it hits the atmosphere inside and vaporizes into hot plasma.
Finding a leak from the inside is difficult because of the phone-booth-size racks of equipment that line many of the station walls. Moving them to search for damage would be cumbersome and time-consuming. “You could be looking behind this rack and that rack, and you’ve lost 30 minutes, and then you find out that [the leak] is behind a rack that takes an hour and a half to get behind because of all the connections,” Studor says. “Meanwhile, we know when the astronauts are going to get hypoxic, and they have to get out of there.” A well-distributed sensor system would give crew members a better chance of triangulating the leak’s location.
Once the damage is detected, assessing it requires an entirely different set of inspection technologies, Studor says. Even when impacts don’t penetrate the station’s hull, they can create pockmarks with sharp edges that can tear an astronaut’s spacesuit during a spacewalk. An actual breach can create cracks that propagate and peel back layers of sharp aluminum around the exit wound. Crew members need to see images of these hazards, as well as assess exactly where the damage has occurred relative to load-bearing parts of the wall.
NASA envisions sending out controllable snake-like robots—essentially hoses that can be manipulated remotely. The snakes would have 3D cameras at their tips and slither into areas of the station where humans can’t go. The idea has been in development for more than a decade: enabling flexibility and dexterity using artificial-muscle technology based on electroactive polymers, which can change shape or size when stimulated by an electric field. But this technology has been difficult to harness, says Yoseph Bar-Cohen, a senior research scientist at the Jet Propulsion Laboratory in Pasadena, California. “Unless they are sizable, you cannot get significant force from these materials,” he says. In 2005, Bar-Cohen staged the first of a series of arm wrestling matches at JPL between humans and a robot arm with electroactive polymers. The robot arms, at least so far, don’t have a chance against their human opponents. “Once we have a winning arm, we know we have it made,” Bar-Cohen says. “It may take a long time.”
Outside the station, assessing damage from an impact presents its own challenges. If the astronauts are sent on a spacewalk to inspect it, they’ll need novel tools. In many cases, Studor says, the damage could be obscured by the very shielding meant to keep it at a minimum. The entry wound might be barely visible. So astronauts may someday use a hand-held backscatter X-ray imager that is based on technology similar to that in airport security detectors. Such imagers would allow astronauts to search for holes by peering into the layers of metal and synthetic fabric shielding. The challenge is making a unit small enough for an astronaut to carry and much more energy-efficient than they are now. It’s much more likely that astronauts would first use a hand-held, camera-tipped borescope, which they could stick down a hole past exterior shielding, he says.
Sending an astronaut outside is always risky, however. That’s why crawling robots—probably the ultimate in spacecraft inspection technology—may find a place on a future space station crew.
Building a robot that can crawl on the outside of a spacecraft in zero gravity without floating away is no simple challenge. One solution, under development at JPL, mimics the anatomy of a gecko, one of nature’s best climbers. The gecko’s sticky feet are marvels of evolution, allowing the animal to cling to walls and seemingly defy gravity. Biologists discovered that microscopic bristles on gecko toes are composed of hundreds and sometimes thousands of even smaller hairs called spatulae. The tiny toe hairs are only about 200 nanometers across (0.2 percent of the width of a human hair), so they interact with the gecko’s traveling surface on a molecular level, through van der Waals forces, a net attractive force that is generated between molecular electron clouds.
JPL engineer Aaron Parness, who became interested in robotics while an undergraduate, began studying these forces in graduate school at Stanford. Wanting to build climbing robots, he and his classmates partnered with biologists at the University of California at Berkeley to study the gecko. For his doctoral project, Parness designed and fabricated synthetic gecko feet.
In 2010 Parness came to JPL, where he and his colleagues have developed synthetic climbers made of a silicon rubber material. The tiny polymer hairs measure 20 microns in diameter—bigger than the gecko’s spatulae but still capable of generating van der Waals forces. The stickiness of the silicon rubber “is based on the geometry of the hairs,” Parness says. “It’s not based on temperature or pressure—the things that would trip you up in space. These van der Waals forces are not sensitive to that.”
A conventional robot crawling along the outside of the station would be confronted with a challenge astronauts know all too well. If you put pressure on a surface to make contact, you push yourself away. But a robot equipped with polymer pads that mimic the adhesiveness of the gecko’s feet should be able to grip whatever it comes in contact with, Parness says.
These robot crawlers could be parked on the outside of the station until they’re called into action to survey damage. That would reduce unnecessary airlock usage.
Parness says that in the short term, spacewalking astronauts could use the technology in a hand-held tool, something the size of a small notebook with a handle on one side and gripper pads on the other. “You could…pull a trigger to get it to stick, and then pull the trigger again to un-stick it and move it to a new spot,” he says.
Parness and his colleagues hope to test a gecko-padded robot prototype on the station in 2016. Today, they’re working on a robot with multiple pairs of pads that can crawl along uneven surfaces and still maintain its grip. An operation like the replacement of a failed cooling pump on the station last December, which required two spacewalks and several hours of dangerous work, may someday be handled by a smart, Spider-Man-like robot, says Parness.